The present invention relates to a temperature sensing device that is provided on a fixing device of an image forming apparatus for controlling the fixing temperature.
Temperature sensing in a fixing device is done by measuring the temperature on the circumferential surface of a fixing roller. In order for temperature sensing to be done accurately, temperature sensing devices which are in a type of direct pressure contact with the fixing roller are used.
The temperature sensing device mentioned above is one that is kept in contact with a rotating fixing roller with an optimum pressure. It is, for example, a temperature sensing device wherein a thermistor temperature sensing element is supported by a supporting member formed of an elastic substance such as foamed silicone or the like, all of which are covered by a protective sheet such as a polyimide film. This temperature sensing device is kept in pressure-contact with a circumferential surface of a fixing roller, through the protective sheet, with a load of 50-150 GMF, thereby the temperature on the roller surface is measured directly, and a heater built in the fixing roller is turned on or turned off by the measured temperature. Thus, the temperature may be controlled.
When a temperature sensing element is supported directly by a supporting member, the temperature on the side of the temperature sensing element where it is in contact with a fixing roller is the same as that on the fixing roller surface, but the temperature on the other side being in contact with the supporting member is different because heat in the temperature sensing element is lost through the supporting member. For example, a temperature difference of 30.degree. C. to 40.degree. C. has been created for the measured temperature of about 190.degree. C., making it impossible to measure accurately the temperature on the fixing roller surface. When setting the fixing temperature to 190.degree. C., the actual controlled temperature on the fixing roller surface can be 200.degree. C.-210.degree. C., far higher than the temperature at which the apparatus is set.
In addition to the above, the temperature difference has tended to be increased by the change of pressure-contact condition of the supporting member caused by friction and vibration caused by on the rotation of the fixing roller.
As one means to overcome the temperature difference, there has been proposed a method wherein a thin metal sheet having a high thermal conductivity such as an aluminum foil is inserted between a temperature sensing element and a supporting member to take advantage of the heat-transfer thereby it is possible to measure and control accurately the temperature on the fixing roller surface.
With regard to the method mentioned above, there has been proposed methods in Japanese Utility Model Examined Publication No. 21315/1989 and Japanese Patent Examined Publication No. 51765/1989. In both cases, however, due to the structure wherein a temperature sensing element is embedded in a cutout and recess having a certain depth provided in advance on a supporting member, the top surface of the temperature sensing element does not always protruded above the supporting member because of the variation of the depth of the cutout and recess of the embedded position, resulting in occasional inability of accurate measurement of temperature on the fixing roller surface, despite transfer the thin metal sheet.
The method suggested in Japanese Utility Model Publication Open to Public Inspection No. 63731/1988, likewise, is not common because it is expensive due to a specific thermistor used therein.
The method suggested in Japanese Utility Model Examined Publication No. 19612/1982 is one wherein a temperature sensing element is attached on a flexible thin layer to be in close-contact therewith and the thin layer is provided with a superficial layer having a lower coefficient of friction. The method, however, is not satisfactory on the point of thermal responding properties.